1. Technical Field
The present disclosure relates to a light receiving device that converts incident light into an electrical signal, and, in particular, to a light receiving device including a semiconductor scanning circuit for reading the signal charge converted from the incident light by a photodiode having a photoelectric conversion function.
2. Description of the Related Art
A light receiving device has been conventionally developed and commercialized, in which a photodiode of a photoelectric converter and a scanning element that transfers photoelectric charges generated by the photodiode are integrated on a semiconductor substrate.
In the conventional light receiving device, the photodiode and the scanning element are disposed on the same plane. Hence, the aperture ratio (the ratio of the amount of light incident on the photoelectric converter to the amount of light incident on the light receiving surface) is small. This results in low light use efficiency and large loss of incident light.
Although development of an on-chip microlens, for example, has increased the substantial aperture ratio, increase in the substantial aperture ratio is limited as long as the photodiode and the scanning element are disposed on the same plane.
In view of the above, a light receiving device has been proposed in which a photodiode for generating photoelectric charges are stacked on the scanning circuit substrate for photoelectric charge transfer.
Since the photodiode serving as a light receiving portion is disposed on the entire surface of the scanning circuit in the light receiving device, the light receiving device can have an aperture ratio close to 100%, which leads to increased sensitivity.
In order to achieve good optical response characteristics, such a light receiving device generally has an electrode which contacts the photodiode in such a manner that charge injection is blocked.
Therefore, in a light receiving device which does not use charge multiplication within the device, it is not possible to take out the signal charges exceeding the number of carriers generated by incident light. This results in the gain of the photoelectric conversion being one or less.
In view of the above, a light receiving device having a photoelectric conversion gain exceeding one, an avalanche multiplication type light receiving device has been developed. In this device, an avalanche multiplication phenomenon is generated by applying a strong electric field to the photodiode to make the gain of the photoelectric conversion one or greater.
In such an avalanche multiplication type light receiving device, the gain which is the ratio of the number of photoelectric charges generated within the photodiode to the number of incident photons ranges from several dozen to several hundred.
The stacked light receiving device described above is formed by forming, on a silicon substrate, a scanning circuit through the semiconductor processes used for a general integrated circuit and sequentially depositing a photodiode and a transparent conductive film on the scanning circuit.
In this case, before the transparent conductive film is formed on the scanning circuit, the scanning circuit is formed through complicated processes performed on a silicon substrate. Hence, it is extremely difficult to smooth the surface of the scanning circuit before the transparent conductive film is formed, which results in that the pixel electrode itself or the boundary of the pixel electrode has unevenness.
Therefore, for example, unlike a photoconductive type image pickup tube where a photoconductive film is formed on a smooth glass substrate, dark current increases due to a local electric field concentration caused by unevenness of the base, which is likely to lead to white spot defects appearing on the screen.
In particular, if it is desired to obtain high sensitivity by using the avalanche multiplication phenomenon in a photodiode, it is necessary to apply a strong electric field to the photodiode. Hence, local dark current injection or avalanche breakdown due to non-uniformity of the electric field is likely to occur.
As a conventional technique for solving the above problems, for example, Japanese Unexamined Patent Application No. H7-192663 (hereinafter, referred to as patent literature (PTL) 1) discloses a structure in which a photoelectric converter, including a transparent conductive film and a photodiode formed on a light transmitting substrate, is connected, via conductive microbumps, to signal reading electrodes of a scanning circuit formed on a substrate different from the light transmitting substrate.
FIG. 9 is a cross-sectional view of a photoelectric converter of a conventional light receiving device. Transparent conductive film 103 and, photodiode 104 are formed on light transmitting substrate 113. First pixel electrodes 105 having a predetermined size are arranged on the surface of photodiode 104 at predetermined intervals. Second pixel electrodes 107 are provided on the surface of scanning circuit 108 at the same pitch as first pixel electrodes 105. Microbumps 106 for electrically connecting photoelectric converter 101 and scanning circuit 102 are provided on second pixel electrodes 107.
As illustrated in FIG. 9, the light receiving device according to the conventional technique has a structure where photoelectric converter 101 and scanning circuit 102 separately formed are electrically connected by microbumps 106 as described above.
In the conventional technique, for example, a substrate which is polished to have a sufficiently flat surface is used. Accordingly, photodiode 104 is formed on a significantly flat base.
Thus, for example, even if a light receiving device is operated by applying, to a photodiode, a high electric field which causes charge multiplication in the photodiode due to an avalanche phenomenon, an increase in dark current or an avalanche breakdown due to local electric field concentration is unlikely to occur.
Moreover, since scanning circuit 102 and photoelectric converter 101 are formed separately, the materials for second pixel electrodes 107 on scanning circuit 108 and for photodiode 104 can be selected without considering the electrical connection characteristics of second pixel electrodes 107 and photodiode 104.
In other words, optimal materials, structures, and manufacturing methods can be used without any constraints imposed by being a stacked image capturing device.
Therefore, in such a stacked structure using the microbumps, for example, as a substrate on which a photodiode is formed, an SOI (Silicon On Insulator) substrate is used which has a silicon oxide film disposed between a silicon substrate and a surface silicon layer. The SOI substrate is effective for a reduction in parasitic capacitance of a transistor, an increase in operating speed, and a reduction in power consumption. Silicon and the silicon oxide film are removed after stacking the scanning circuit and the microbumps, and a transparent conductive film is formed. In this way, the characteristics of the photodiode can be further improved.